Caenorhabditis elegans is a transparent nematode (roundworm) about 1mm in length. Since the 1970s, C. elegans has become the workhorse of many biological disciplines. Owing to its simplicity, C. elegans was the first multicellular organism to have its genome completely sequenced. In addition, its transparent body has allowed biologists to map out the spatial localtion, as well as the developmental fate of each of its 959 somatic cells. Incredibly, almost a third of these cells are neurons. C. elegans is also, to date, the only organism whose connectome (or pattern of neuronal connectivity) has been mapped. In addition, laser ablation experiments have been used to study the behavioural consequences of selectively lesioning individual neurons, so many neurons can be assigned a functional label. This makes it an obvious gold standard for testing network methods before applying them to larger nervous systems (or human brains) where the validation and interpretation of results is much more controversial. Recently, I have applied this approach in several projects in collaboration with: William Schafer, Barry Bentley, Chris Barnes, Vito Latora, Vincenzo Nicosia, Sebastian Ahnert, Emma Towlson, Ed Bullmore, Gang Yan and Laszlo Barabasi.
The first port of call for anyone wanting to learn about this very special creature is without a doubt the WormAtlas.
For those interested in the C. elegans connectome from a networks perspective, I would recommend the following review paper as a starting point: Structural Properties of the Caenorhabditis elegans Neuronal Network. Varshney, Chen, Paniagua, Hall and Chklovskii. (2011) PLoS Comp Bio